Several slide-casting machines for hollow slabs are known in prior art, which are of a similar principle as compared with each other and in which the concrete mix is extruded in the machine by means of spiral screws. The machine runs along rails placed on the base. The spiral screw is of conical shape with the cone expanding towards the final end, whereby an efficient compacting of the concrete is also achieved.
Immediately as an extension of the spiral screw, there is a shaping member, i.e. a so-called cavity mandrel, which is vibrated by means of a vibrator fitted inside the mandrel. Moreover, a vibrator beam fitted in the deck portion of the machine is vibrated, wherein the vibration of the cavity mandrels together with the surface vibration at the top of the machine produces an ultimate compacting of the concrete.
The shaping member is typically provided for forming a cavity of circular cross-sectional shape. However, it is also known to form the cavity in any of several non-circular cross-sectional shapes. Such an apparatus is shown in U.S. Pat. No. 3,877,860, to Putti.
The cavity mandrel is followed by a so-called follower tube, whose function is to support the cavity wall at the final end of the machine.
Drawbacks of the cavity mandrel are the strong noise (higher than 85 dBA) resulting from the high vibration frequency, the high power requirement, and the low efficiency of the vibration power used for the vibration.